Opioids such as morphine are mainstay treatments for clinical pain conditions. Itch is a common side effect of opioids, particularly as a result of epidural or intrathecal administration. Recent progress has advanced our understanding of itch circuits in the spinal cord. However, the mechanisms underlying opioid-induced itch are not fully understood, although an interaction between µ-opioid receptor (MOR) and gastrin-releasing peptide receptor (GRPR) in spinal GRPR-expressing neurons has been implicated. In this study we investigated the cellular mechanisms of intrathecal opioid-induced itch by conditional deletion of MOR-encoding Oprm1 in distinct populations of interneurons and sensory neurons. We found that intrathecal injection of the MOR agonists morphine or DAMGO elicited dose-dependent scratching as well as licking and biting, but this pruritus was totally abolished in mice with a specific Oprm1 deletion in Vgat+ neurons [Oprm1-Vgat (Slc32a1)]. Loss of MOR in somatostatin+ interneurons and TRPV1+ sensory neurons did not affect morphine-induced itch but impaired morphine-induced antinociception. In situ hybridization revealed Oprm1 expression in 30% of inhibitory and 20% of excitatory interneurons in the spinal dorsal horn. Whole-cell recordings from spinal cord slices showed that DAMGO induced outward currents in 9 of 19 Vgat+ interneurons examined. Morphine also inhibited action potentials in Vgat+ interneurons. Furthermore, morphine suppressed evoked inhibitory postsynaptic currents in postsynaptic Vgat− excitatory neurons, suggesting a mechanism of disinhibition by MOR agonists. Notably, morphine-elicited itch was suppressed by intrathecal administration of NPY and abolished by spinal ablation of GRPR+ neurons with intrathecal injection of bombesin-saporin, whereas intrathecal GRP-induced itch response remained intact in mice lacking Oprm1-Vgat. Intrathecal bombesin-saporin treatment reduced the number of GRPR+ neurons by 97% in the lumber spinal cord and 91% in the cervical spinal cord, without changing the number of Oprm1+ neurons. Additionally, chronic itch from DNFB-induced allergic contact dermatitis was decreased by Oprm1-Vgat deletion. Finally, naloxone, but not peripherally restricted naloxone methiodide, inhibited chronic itch in the DNFB model and the CTCL model, indicating a contribution of central MOR signalling to chronic itch. Our findings demonstrate that intrathecal morphine elicits itch via acting on MOR on spinal inhibitory interneurons, leading to disinhibition of the spinal itch circuit. Our data also provide mechanistic insights into the current treatment of chronic itch with opioid receptor antagonist such as naloxone.

Tendon injury produces intractable pain and disability in movement, but the medications for analgesia and restoring functional integrity of tendon are still limited. In this study, we report that proteinase-activated receptor 2 (PAR2) activation in dorsal root ganglion (DRG) neurons contributes to chronic pain and tendon histopathological changes produced by Achilles tendon partial transection injury (TTI). Tendon partial transection injury increases the expression of PAR2 protein in both somata of DRG neurons and their peripheral terminals within the injured Achilles tendon. Activation of PAR2 promotes the primary sensory neuron plasticity by activating downstream cAMP-PKA pathway, phosphorylation of PKC, CaMKII, and CREB. Blocking PAR2 signaling by PAR2 small-interference RNA or antagonistic peptide PIP delays the onset of TTI-induced pain, reverses the ongoing pain, as well as inhibits sensory nerve sprouting, and promotes structural remodeling of the injured tendon. Vitamin B complex (VBC), containing thiamine (B1), pyridoxine (B6), and cyanocobalamin (B12), is effective to ameliorate TTI-induced pain, inhibit ectopic nerve sprouting, and accelerate tendon repair, through suppressing PAR2 activation. These findings reveal a critical role of PAR2 signaling in the development of chronic pain and histopathological alterations of injured tendon following Achilles tendon injury. This study suggests that the pharmaceuticals targeting PAR2, such as VBC, may be an effective approach for the treatment of tendon injury–induced pain and promoting tendon repair.

Nerve injury causes hyperexcitability of the dorsal root ganglion (DRG) and spinal dorsal horn (DH) neurons, which results in neuropathic pain. We have previously demonstrated that partial dorsal rhizotomy (PDR) produced less severe pain-like behavior than chronic constriction injury (CCI) or chronic compression of DRG (CCD) and did not enhance DRG neuronal excitability. However, the mechanisms underlying such discrepancy remain unclear. This study was designed to compare the activation of phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) in DRG and DH, and c-Fos in DH following treatments of CCI, CCD, and PDR. We confirmed that thermal hyperalgesia produced by PDR was less severe than that produced by CCI or CCD. We showed that pERK1/2 in DRG and DH was greatly activated by CCI or CCD, whereas PDR produced only transient and mild pERK1/2 activation. CCI, CCD, and PDR induced robust c-Fos expression in DH; nevertheless, c-Fos⁺ neurons following PDR were much fewer than that following CCI or CCD. Blocking retrograde axonal transport by colchicine proximal to the CCI injury site diminished thermal hyperalgesia and inhibited pERK1/2 and c-Fos activation. These findings demonstrate that less severe pain-like behavior produced by PDR than CCI or CCD attributes to less activation of pERK1/2 and c-Fos. Such neurochemical activation partially relies on retrograde axonal transport of certain “injury signals” from the peripheral injured site to DRG somata.

Understanding the neural mechanisms underlying anesthetic-induced unconsciousness remains a central question in both anesthesia research and neuroscience. However, the role of non-neuronal components in modulating anesthetic sensitivity is poorly understood. In this study, we identify a microglia-dependent platelet-derived growth factor subunit B (PDGFB) signaling pathway that modulates propofol anesthesia sensitivity and contributes to the induction of anesthesia in mice. We demonstrate that PDGFB signaling regulates the potassium voltage-gated channel subfamily D member 2 (KCND2) in the hypothalamic paraventricular nucleus (PVN) in mice, thereby orchestrating the excitability of PVN glutamatergic neurons (PVN ^Glu ) and the activity of their projections to lateral hypothalamic glutamatergic neurons (LH ^Glu ). Microglia deprivation induces PDGFB deficiency and downregulates KCND2, resulting in PVN hyperexcitability and hyperactivation of the PVN → LH arousal circuit, which contributes to resistance to anesthesia induction in mice. Our findings identified non-neuronal molecular mechanism that governs anesthetic sensitivity, complementing the classical γ-aminobutyric acid type A receptor-centric model and providing insights into the regulation of anesthesia induction.

Opioids are the primary treatment for severe pain in clinical practice. However, repeated exposure to opioids leads to tolerance, and underlying mechanisms remain elusive. The present study demonstrates that EphB1 receptors play an important role in opioid antinociception through orchestrating µ‐opioid receptor (MOR) trafficking. Morphine administration activates EphB1 receptors in the dorsal root ganglion (DRG) and the spinal cord. Deletion of EphB1 receptors in DRG neurons or vesicular glutamate transporter 2 (Vglut2)‐expressing neurons in the spinal dorsal horn (DH) augments morphine antinociception. MOR and EphB1 receptors are co‐expressed in the somata and axons of DRG neurons and DH Vglut2 ⁺ neurons. Deletion of EphB1 receptors in DRG neurons increases opioid‐induced suppression of calcium currents, and ablation of EphB1 receptors in Vglut2 ⁺ neurons enhances opioid‐induced inhibition of excitatory synaptic transmission and induction of outward currents in Vglut2 ⁺ DH neurons. Mechanistically, EphB1 receptors form a complex with MOR and G protein‐coupled receptor kinase 2 (GRK2) and regulate GRK2‐mediated phosphorylation of MOR at Ser ³⁷⁵ , thus controlling MOR internalization. These findings support the idea that activation of EphB1 receptors may facilitate the development of opioid tolerance through regulating MOR.